Monoclonal antibody which has been vastly developed as an antibody pharmaceutical product in recent years is a protein multimer consisting of H chain and L chain. Many improving techniques have been proposed in an attempt to enhance its drug efficacy and the like. For example, it has been shown that shuffling of the genes of H chain and L chain of an antibody is effective for improving the affinity of the antibody. That is, an antibody protein (e.g., Fab-type antibody) which is translated from the gene of either H chain library or L chain library and the corresponding partner chain are displayed in an in vivo selection system such as phage display (non-patent document 1: J. Mol. Biol., 1996.255.28-43, non-patent document 2: Biochem. Biophys. Res. Commun., 2002.295.31-36), yeast surface display (non-patent document 3: Protein Eng. Des. Sel., 2010.23.311-319) and the like, and selections are performed based on the affinity for the target antigen, whereby a clone with a higher affinity can be selected. In phage display and yeast surface display, however, a preparation of a library having diversity exceeding 1010 molecules is so hard that it requires substantial efforts and time, since the insertion efficiency of gene into an expression vector and the transformation efficiency of host cells are limited (generally 107-1010 molecules).
On the other hand, an in vitro selection system utilizing an in vitro translation system, which is represented by ribosome display (patent document 1: JP-A-2008-271903), is a technique for forming a complex of a gene (mRNA or cDNA) and a protein encoded by the gene in the in vitro translation system, and efficiently selecting a protein having a particular function. The essential element here is that the genotype and the phenotype correspond one-to-one. Specifically, cDNA or mRNA as a genotype needs to be physically bound one-to-one with a protein translated therefrom. Since a complex having a desired function is selected based on the properties of a protein phenotype, the number of the complexes shows the diversity of the library. Preparation of a complex in an in vitro selection system does not require a step of transformation of living cells, which is necessary for the in vivo selection system such as yeast surface display and the like, and can be achieved by the addition of a gene library to the in vitro translation system. Therefore, a library of complexes exceeding 1013 molecules can be easily prepared in the in vitro selection system, and a desired functional molecule can be selected using the library.
As mentioned above, the in vitro selection system is constituted by forming a one-to-one complex of a gene (mRNA or DNA) and a protein encoded by the gene by adding a gene library to an in vitro translation system. Therefore, if the protein to be selected is a protein multimer consisting of plural gene products, it is difficult to prepare a gene-protein complex.